ii6 Vertebrate Embryology 



folds may be formed. If these folds, the head, 

 tail, and lateral, be pushed under far enough 

 they will meet under the centre of the bottle, 

 and we shall have the bottle, with its surround- 

 ing layers of cloth, connected with the rest of 

 the model by only a sort of stalk, which is 

 hollow and is composed of the three layers of 

 cloth {cf. Fig. 37, H and L). The bottle is 

 used simply to give a solid object around which 

 the folding may more easily be done, but we 

 are to consider the space occupied by the bottle 

 as an empty space. 



We have now represented what is sometimes 

 called the embryo-sac, or simply the embryo, in 

 contradistinction to Xh^. yolk-sac, or simply the 

 yolk. The embryo remains connected with 

 the yolk throughout the period of incubation 

 by the yolk- or somatic-stalk, and as the embryo 

 increases in size, the yolk-sac is, by absorption, 

 constantly diminished (Fig. 37). The space 

 occupied by the bottle, in our model, repre- 

 sents the digestive tract of the chick, and is 

 lined, as will be seen by examination of the 

 model, by the lower germ layer or entoblast. 

 The body-cavity would be difficult to represent 

 in the cloth model, but it can be imagined to 

 exist as the narrow space between the two 



